1. Field of the Invention
This invention relates to heating and cooling systems and, more specifically, to controlling heating and cooling systems.
2. Background Information
An air conditioner typically has at least four major components. Those components are a compressor, a condenser, a throttling device (such as a fixed orifice, a thermal expansion valve, or an electronic expansion valve), and an evaporator.
During normal operation, the compressor pumps the refrigerant from a low-pressure, low-temperature vapor state to a high-pressure, high-temperature vapor state. The hot refrigerant gas passes through the condenser, which is generally located outside in communication with ambient air. The condenser rejects heat to the ambient air typically though the use of an electric fan. The refrigerant leaving the condenser is normally in a sub-cooled liquid state. The liquid refrigerant then flows though the throttling device with a significant pressure drop. In particular, the throttling device controls the refrigerant's mass flow by regulating the degree to which the valve is opened. The refrigerant leaving the throttling device is in a saturated state where vapor and liquid coexist. The saturated refrigerant passes through the evaporator. A blower typically forces indoor air past the evaporator to cool the indoor air. As the indoor air heats up the refrigerant, the refrigerant evaporates into a vapor state. If all refrigerant is evaporated in the evaporator, the refrigerant returns to the compressor as superheated gas. Otherwise, liquid refrigerant may enter the compressor causing damage to the compressor, as liquid is largely incompressible. Superheat refers to the temperature of the vapor-state refrigerant above its boiling point.
The entry of two-state refrigerant into the compressor is more likely to occur upon startup of the air conditioner and at other times when a steady state has not yet been established (a transient state), such as a fan speed change that alters the heat transfer from the condenser or evaporator. Prior art EXV implementations generally maintain an appropriate flow of superheated gaseous refrigerant to the compressor during steady state operation, but such EXVs typically do not react quickly to transient states. While an EXV can be controlled to respond to a startup condition or fan speed change by applying a preprogrammed adjustment, there is still the possibility that some liquid-state refrigerant may enter the compressor. This is because the EXV is adjusting its flow based upon various predictions for refrigerant performance. The predictions may not take into account all circumstances (for example, colder inlet air, humidity levels, a non-standard refrigerant charge, etc). The compressor's life, moreover, can be shortened even if only a small amount of incompressible liquid enters the compressor and/or only momentarily.
In addition, the dramatic change in refrigerant superheat level during startup and transient states was often handled by a complete opening or closing of the EXV. This shortens valve life.
Thus, a need exists to ensure that all refrigerant entering the compressor is in the gaseous state and free of liquid-state constituents, and to increase EXV life and improves system performance under all operating conditions.